1. Field of the Invention
The present invention relates to a method, apparatus, and system for optical amplification, and more particularly to a method, apparatus, and system for optical amplification suitable for WDM signal light obtained by wavelength division multiplexing a plurality of optical signals allocated to different wavelength channels.
2. Description of the Related Art
An optical communication system using an optical fiber transmission line is used to transmit a relatively large amount of information. A low-loss (e.g., 0.2 dB/km) optical fiber has already been produced and is being used as the optical fiber transmission line. In addition, an optical amplifier for compensating for loss in the optical fiber transmission line is used to allow long-haul transmission.
A related art optical amplifier includes an optical amplifying medium pumped by pump light to provide a gain band. The optical amplifying medium and the pump light are selected so as to provide a gain band including the wavelength of signal light to be amplified. As a result, the signal light is amplified during propagation in the optical amplifying medium being pumped. For example, an erbium doped fiber amplifier (EDFA) includes an erbium doped fiber (EDF) as the optical amplifying medium, and a pumping light source for pumping the EDF. The pumping source supplies pump light having a predetermined wavelength to the EDF. By presetting the wavelength of the pump light within a 0.98 xcexcm band or 1.48 xcexcm band, a gain band including a wavelength band of 1.55 xcexcm can be obtained. As a result, signal light having a wavelength band of 1.55 xcexcm is amplified.
Another type of the related art optical amplifier has a semiconductor chip as the optical amplifying medium. In this case, pumping is performed by injecting an electric current into the semiconductor chip.
As a technique for increasing a transmission capacity by a single optical fiber, wavelength division multiplexing (WDM) is known. In a system adopting WDM, a plurality of optical carriers having different wavelengths are individually modulated by data. Each modulated carrier provides one channel of a WDM system for transmitting optical signals. These optical signals (i.e., the modulated carriers) are wavelength division multiplexed by an optical multiplexer to obtain WDM signal light. The WDM signal light thus obtained is transmitted through an optical fiber transmission line to a receiving end. At the receiving end, the WDM signal light is separated into individual optical signals by an optical demultiplexer. Then, the original data can be detected according to these individual optical signals. Accordingly, by applying WDM, the transmission capacity in a single optical fiber can be increased according to the number of WDM channels.
In the case that an optical amplifier is inserted in a transmission line of an optical communication system adopting WDM, a transmission distance is limited by the wavelength characteristic of gain (wavelength dependence of gain) which is represented by a gain tilt or gain deviation. In an EDFA, for example, it is known that a gain tilt is produced at wavelengths near 1.55 xcexcm and that this gain tilt varies with the total input power of signal light to the EDFA and the power of pump light.
Known is an optical amplifier for optical amplification which can maintain the wavelength characteristic of gain constant and obtain a wide input dynamic range. This optical amplifier includes first and second optical amplifying sections and a variable optical attenuator optically connected between the first and second optical amplifying sections. Automatic gain control (AGC) is applied to each of the first and second optical amplifying sections, thereby maintaining constant the wavelength characteristic of gain of each of the first and second optical amplifying sections. Further, automatic output level control (ALC) is performed by using the variable optical attenuator to thereby obtain a wide input dynamic range. That is, the output level of the second optical amplifying section is maintained constant irrespective of the input level of the first optical amplifying section, so that the input dynamic range of this optical amplifier is widened.
In such an optical amplifier, the attenuation of the variable optical attenuator is controlled so that the power per channel of WDM signal light to be supplied to the second optical amplifying section becomes constant, for example. Further, the output power of an internal pumping source in the second optical amplifying section is controlled to maintain the gain constant. It is known that if the number of channels of WDM signal light to be amplified changes or is modified, the power of a pumping source for obtaining a constant gain increases with an increase in total power of light to be amplified. Owing to this fact, the output power of the internal pumping source may fall outside of an adjustable range. For example, when the number of channels of WDM signal light to be amplified increases in concert with system upgrading, there is a possibility that the output power of the internal pumping source may exceed the adjustable range. However, the related art method cannot easily respond to such a change in the number of channels of WDM signal light.
It is therefore an object of the present invention to provide a method, apparatus, and system for optical amplification which can easily respond to a change in the number of channels of WDM signal light.
Other objects of the present invention will become apparent from the following description.
In accordance with a first aspect of the present invention, there is provided a method for optical amplification. An optical amplifying module having an internal pumping source is provided. WDM signal light obtained by wavelength division multiplexing a plurality of optical signals allocated to different wavelength channels is supplied to the optical amplifying module. An output power from the internal pumping source is adjusted so that the optical amplifying module provides a gain for the WDM signal light. A booster module having an external pumping source for supporting the gain is connected to the optical amplifying module. An output power from the external pumping source is set according to the number of channels of the WDM signal light.
According to this method, although the number of channels of the WDM signal light has changed, the output power from the external pumping source is set according to the number of channels, thereby supporting the gain given to the WDM signal light by the optical amplifying module. Accordingly, this method can easily respond to a change in the number of channels of the WDM signal light, thus achieving one of the objects of the present invention.
Preferably, the power per channel of the WDM signal light to be supplied to the optical amplifying module is maintained constant. In this case, the power (e.g., total power) of the WDM signal light to be supplied to the optical amplifying module may be detected and the number of channels of the WDM signal light may be obtained according to the power detected above.
Preferably, the gain of the optical amplifying module is detected, and the output power from the internal pumping source is controlled so that the gain detected is maintained constant. The adjustable range of the output power from the internal pumping source is predetermined according to the capacity of the internal pumping source. On the other hand, an optimum output power from the internal pumping source for maintaining the gain constant increases with an increase in the number of channels of WDM signal light to be amplified. Accordingly, by applying the present invention, such control for maintaining the gain constant can be easily performed.
In accordance with a second aspect of the present invention, there is provided a method comprising the steps of (a) providing an optical amplifying module having an internal pumping source; (b) supplying signal light to the optical amplifying module; (c) adjusting an output power from the internal pumping source so that the optical amplifying module provides a gain for the signal light; (d) connecting a booster module having an external pumping source to the optical amplifying module; and (e) setting an output power from the external pumping source according to the power of the signal light.
In this method, the signal light to which the gain is provided by the optical amplifying module is not limited to WDM signal light. Also in the case that the signal light is an optical signal of only one channel, there is a possibility that the output power from the internal pumping source may fall outside the adjustable range because of a change in power of the optical signal. Accordingly, by setting the output power from the external pumping source according to the power of the signal light, the output power can easily respond to a change in power of an optical signal to be amplified.
In accordance with a third aspect of the present invention, there is provided an apparatus comprising an optical amplifying module having an internal pumping source, to which WDM signal light obtained by wavelength division multiplexing a plurality of optical signals allocated to different wavelength channels is supplied; an adjusting unit for adjusting an output power from the internal pumping source so that the optical amplifying module provides a gain for the WDM signal light; and a booster module having an external pumping source for supporting the gain, the booster module being connectable to the optical amplifying module; an output power from the external pumping source being set according to the number of channels of the WDM signal light. According to the third aspect of the present invention, it is possible to provide a suitable apparatus for carrying out the method according to the first aspect of the present invention.
In accordance with a fourth aspect of the present invention, there is provided a system comprising an optical fiber transmission line and at least one optical repeater arranged along the optical fiber transmission line. Each optical repeater may include the apparatus according to the third aspect of the present invention. According to the fourth aspect of the present invention, it is possible to provide a suitable system for carrying out the method according to the first aspect of the present invention.
In accordance with a fifth aspect of the present invention, there is provided an apparatus comprising a front-stage optical amplifying section and a rear-stage optical amplifying section each for providing a gain to WDM signal light obtained by wavelength division multiplexing a plurality of optical signals allocated to different wavelength channels; a variable attenuator optically connected between the front-stage optical amplifying section and the rear-stage optical amplifying section; a circuit for controlling the variable attenuator so that the power per channel of the WDM signal light to be supplied to the rear-stage optical amplifying section is maintained constant; and a booster module having an external pumping source for supporting the gain in the rear-stage optical amplifying section; an output power from the external pumping source being set according to the number of channels of the WDM signal light.
The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.